The present invention relates to grindstones which are used in the finishing of the steel material of bearing race inner surfaces, rolling elements, and other types of sliding surfaces.
Typically, a plateau surface is required in the inner surfaces of roller bearing races, rolling elements, and other types of sliding surfaces from the point of view of the coefficient of friction, heat generation, lubricating oil retention, and seizure-resistant service life. Superfinishing grindstones have been developed to form these plateau surfaces, as recited in Laid-open Japanese Patent No. 3-239475. The superfinishing grindstone disclosed in the above patent essentially uses either silicon carbide, fused alumina abrasive grains or mixtures of these to a maximum of 80-95% of the volume when the whole volume of these grains of a grindstone is set at 100, and cubic boron nitride abrasive grains with an average grain size of 5-30 times over the mean abrasive grain size for the remaining 5-20%.
Further, Laid-open Japanese Patent No. 2000-343438 proposes a vitrified grindstone that contains carbonized bead-shaped grains which are used as a solid lubricant and not as the abrasive. They reduce cutting resistance and increase the cutting ability of diamond and CBN (cubic boron nitride) abrasives.
However, a detailed electron micrographic study of surfaces of steel material of bearing race inner surfaces, rolling elements, and other types of sliding surfaces which have been finished with conventional superfinishing grindstones revealed abrasive grains still stuck into the surface of sliding surfaces, and particularly alumina abrasive grains which were stuck in the finished surface or which had fallen out and left scratches in the surface.
No matter how small the abrasive grains may be, abrasive grains left sticking into the finished sliding surface are hard to remove by washing. Also, the hard abrasive grains can fall out while the bearing is in use, can become lodged between the sliding surfaces, and can seriously impede the performance of the bearing. The result is an increase in the production of defective parts.